1. Field of the Invention
The invention relates to apparatus for locating objects behind a wall lining, and in particular wall framing such as studs. Such apparatus are commonly known as stud detectors. Such devices have also been developed to detect other objects within wall, ceiling and floor structures, such as AC wiring and metal pipes and rods. The invention also relates to a method for locating objects behind a wall lining.
2. Description of Prior Art
Wooden stud detectors are well known and rely on detecting changes in dielectric constants in capacitive sensors. Stud detectors are typically used by tradesmen to detect wall framing, such as studs or beams, that are hidden from view behind plaster or other wall coverings, by moving the detector across and closely adjacent a wall covering. A portable stud locating device is described in U.S. Pat. No. 4,099,118.
Known devices operate by detecting a change, or imbalance, in a capacitive sensor element which results from the presence of a stud, or other wooden object, behind the wall covering. A number of other factors can affect the capacitive sensor and thus the unit must be calibrated before each use. Factors that affect the capacitive sensor include the thickness of the wall covering and the presence of the operator's hand on the device.
In the device described in U.S. Pat. No. 4,099,118 calibration is implemented by charging a capacitor of high input impedance. However, capacitors do not maintain their charge indefinitely and charge leakage causes a loss of calibration. To ameliorate this problem, known stud detectors use comparatively expensive capacitors with a very low charge leakage. Even with an expensive low leakage capacitor extended use of the device leads to deviation of performance due to changes in calibration.
Furthermore, it can take several seconds to charge the capacitor to calibrate the device and there is a need for an improved calibration circuit to reduce the calibration time and thus improve the user-friendliness of such devices.
Another problem is that these devices need be easy to hold and use as any movement of the operator's hand will affect the calibration of the device.
It is often desirable to detect objects in difficult locations such as behind ceiling coverings, at the edges and corners of walls and behind objects in a room. This means that the device will possibly be held in a variety of orientations and moved in a variety of patterns during operation. The user may not have a direct line of sight to the indicators on the device and thus and is desirable to have indicators which are quickly and readily discernible.